MP stands for multiprocessor. If you're building a dual-processor system, that's what the MP version is there for (although folks often use modified XP's for that purpose).
The 2600+ would be a decent mid-range gaming CPU, pair it up with PC2700 memory (or higher) and an nForce2 board, but make sure to get yourself a new video card. Besides obvious performance reasons, you must not plug an AGP 2X or older video card into a modern motherboard or Bad Things Will Happen To You. :Q Suggestion: Radeon 9800 Pro 128MB, and if you wait a bit the price should take enough of a drop to just about pay for...
...your new power supply. Unless your PSU is newer than your K6-2 system, you'd better arrange for some sort of respectable 350W+ unit like an Enermax 365-series unit or Antec SL350, perhaps. Fortron is getting some good remarks too, they've got units with big 120mm fans for low noise levels.
Shuttle's AN35N Ultra is a decent budget board if you don't need Firewire or hardware-accelerated audio (it has audio, just not fancy hw-accelerated audio). Abit's NF7-S is the overclockers' sweetheart around here, and has Firewire, RAID, SATA and hw-accelerated audio too.
Hope that helps... welcome to the Forums
